Note: for those who may be confused as to what is a flannel board, it's literally a board covered in flannel. There are many versions available, and below is the one that I use complete with pockets.

My flannel making technique in four easy steps:
1) Find a coloring page online or draw an image yourself
2) Color image (or outsource to your library volunteers)
3) Cut out and laminate pictures. You might be fancy enough to have access to a laminating machine. As I don't, I use KAPCO, a hard plastic designed to cover books, which also works wonderfully for this purpose.
4) Stick a small square of sandpaper to the back using double stick tape.
Voila! The pictures will stick to the flannel board, and I find them much more aesthetically pleasing than using flannel fabric with sharpie markings.
The flannels I use most often are "Little Red Wagon" and "Baa Baa Black Sheep". Both have the same concept -- a color themed nursery rhyme/song with an interactive element of guessing which color we'll sing about next. After first singing about the title color, I keep the rest of the options in the pocket of the flannel board and have children predict which color will be chosen at random. Once we've sang with three or four different colors, we'll wave goodbye to each color as a further way to practice color names. As in, "bye bye, blue wagon" or "bye bye, yellow sheep" and so on.
This is the "Little Red Wagon" flannel:

And the "Baa Baa Black Sheep" flannel:

A seasonal flannel I adore is "Five Little Snowmen", just because it's rather fun to act out the melting of each snowman. You can even display the lyrics for parents. Last time I sang this song one observant child asked me why there was one snowman whose nose wasn't colored black. Good question!

Some further resources you can use for flannel ideas are The Flannel Board Storytelling Book by Judy Sierra (H.W. Wilson, 1997), Storytime Magic: 400 Fingerplays, Flannel Boards and Other Activities by Kathy MacMillan and Christine Kirker (ALA, 2009) and Flannelboard Stories for Infants & Toddlers by Ann Carlson (ALA, 2005).